Only add "l" to XX.htm URLs if XX.html exists

Fixes esp8266#6792

Only append the "l" to ".htm" when there actually is a ".html" file
present.  If not, fall-thru like before and try .gz through the normal
paths.

Author: earlephilhower

libraries/ESP8266WebServer/src/detail/RequestHandlersImpl.h

@@ -102,7 +102,7 @@ class StaticRequestHandler : public RequestHandler<ServerType> {
             // Append whatever follows this URI in request to get the file path.
             path += requestUri.substring(_baseUriLength);
 
-            if (!_fs.exists(path) && path.endsWith(".htm")) {
+            if (!_fs.exists(path) && path.endsWith(".htm") && _fs.exists(path + "l")) {
                 path += "l";
             }
         }